1. Introduction

Shale rocks are sedimentary formations which belong to the category of mudstones because they result from the compaction of mud mineral particles over very large timescales. During the last decades, and besides being a potential source of non-conventional hydrocarbons (Dyni and Qian 2006), black shalerocks have been explored to prove their suitability as geological media for long term radioactive waste disposal (Frank et al. 2010; Christopher 2014). Indeed, their specific properties such as low permeability, self-sealing, the prevailing chemically reducing conditions and their high capacity of sorption make the shale formations ideal candidates to permanently host high level radioactive wastes repositories.

Morocco has three major black shale deposits, namely Tanger, Tarfaya and Timahdit belonging to the Upper Cretaceous, 70 Myrs, which represent about 15% of known black shale reserves in the world (Ambles et al. 1988). Many studies have been carried on these black shale, among them chemical characterization of kerogen (Ambles et al. 1988) and composition and physicochemical properties (Saoiabi et al. 2001). The main chemical studies concerning the black shale from Timahdit involved its pyrolysis (Nuttall et al. 1983) and its thermal degradation (Alaoui-Sosse et al. 1988). On the other hand, black shale from the Tarfaya basin were explored for their geochemical characterization (Kolonica et al. 2002), and the work was aimed at producing new adsorbents and testing them for their potential in trapping of radionuclides such as U and Th isotopes (Bekri et al. 1991; Khouya et al. 2002; Galindo et al. 2007). The Ama Fatma is located in the basin of Tarfaya-Boujdour. The basin corresponds to the section of the Atlantic margin between the town of Tarfaya in the north and that of Boujdour in the south. It is limited in the west by the Atlantic Ocean and in the east by the Republic of Mauritania (Figure 1). This area is located on the road leading to Tarfaya, about 80 km north of the city. This section begins with black shale thicker than 2 m (Figure 2) (El Batal 2014), alternating with grayish to blackish limestones, The Ama Fatma cut has been dated thanks to numerous ammonites, it is of lower Turonian age. The black shale of Ama Fatma, which is the subject of the present study, has been found to be rich enough of organic matter chemically linked to a mineral matrix (Khouya 2002). Organic deposits were previously interpreted as the result of early coastal upwellings (Einsele and Wiedmann 1982).

Figure 1. A sketched map of the study area showing its main geological features (Gebhardt and Zorn 2008)

Figure 2. Pictures and schematic profile showing the stratigraphic layers of the studied location (El Batal 2014)

In the framework of the research being carried out on natural matrices for the long storage of radioactive wastes, the main objective of this study was to characterize the rock shale samples, collected from the Ama Fatma location belonging to the Tarfaya bassin, form a mineralogical and chemical point of view. This included the mineralogical composition, major element geochemistry, trace elements enrichment and their subsequent use in redox conditions reconstruction.

2. Material and Methods

2.1. Characterization

Prior to characterization and geochemical analyses, the collected black shale samples (El Batal 2014) were ground to fine powder using electric grinder. The mineralogy of the samples was determined in the National Center of Scientific and Technical Research (CNRST) by X-ray diffractometry using The X’Pert Pro MPD PAnalytical 2-circle diffractometer installed equipped with a thêta-thêta goniometer, a rapid X’Celerator detector, a sample changer and a high temperature chamber. Particle size distributions were determined in the National Center for Energy Sciences and Nuclear Techniques (CNESTEN- Morocco) in the homogenized sub-samples using a wet Laser Diffraction equipment (Malvern Mastersizer 2000) using the Hydro 2000G dispersion Unit. The particle size distribution range is 0.02–2000 µm. A small amount of sample (1g) was introduced in the dispersion unit containing demineralized water, used as dispersant, and then measured after a brief time (10s) of ultrasound application to disperse any agglomerates.

2.2. Determination of Major and Traces Elements

Major elements data were obtained by using X-ray fluorescence (XRF) S1 Turbo SD spectrometer. For trace elements determination, 50 mg of the fine fraction of each sample was weighed into acid-cleaned Teflon vessels, and 3 ml of HNO₃ Supra pure and 5 ml of HF were added. The mixture was allowed to react overnight and then subjected to microwave digestion Speed Wave Four (Berghof technologies). The obtained solubilized samples were analyzed by ICP-MS using a Thermo Scientific XSERIES 2.

3. Results and Discussion

3.1. Black Shale Characterization

From Table 1, the granulometric distribution showed a predominance of sand (50.42 – 83.76%), followed by silt (16.13 – 30.18%) and then clays with contents ranging from 0.1% to 19.40% with the highest value recorded at the upper layer, S1(9). Consequently, the black shale samples, subject of this study, have predominantly a sandy loam texture.

Table 1. Particle size and mineralogical composition of the black shale from the Ama Fatma site.TOC contents were determined by El Batal (2014)

In Table 1, the mineralogical composition, obtained by X-ray diffraction analysis, is given. Calcite is the major constituent of the black shale of the Ama Fatma site. The percentages of calcite were ranging from 47.9 to 70.2%, while other minerals such as dolomite (4.7 à 20.6%), Quartz (3.4 à 17.5%), clays (3.7 – 8.3%), halite (2.7 – 7.3%), fluorapatite (0.4 – 13.3%) and pyrite (0.3 – 9.7%) were relatively less abundant.

3.2. Major Elements Geochemistry

Major elements were analyzed in our sample in order to use them in conjunction with the mineralogical data to establish the element/mineral association. Although elements associations could vary from one sample to another, a correlation analysis would indicate the general trends (Fu et al. 2010a). Major elements concentrations are presented in Table 2. CaO, whose concentration varied between 27.5 – 71.05%, was the most abundant in our samples, followed in importance by silicon and iron oxides, SiO₂ (3,09-22,4%) and Fe₂O₃ (4,50-15,77%), while Al₂O₃ (0,53 - 8% ), Na₂O (1,52-4,06%) and MgO (1,09-4,51%) were less abundant. Other elements such as P₂O₅, K₂O, TiO₂ and MnO were present in weak concentrations, often less than 1%. The average concentrations of Al₂O₃, K₂O, TiO₂ and MgO were similar to those reported for the widely used geochemical standards; the Upper Continental Crust (UCC; Taylor and McLennan 1985), Post Archean Australian Shale (PAAS; Taylor and McLennan 1985) and North American Shale Composite (NASC; Gromet et al. 1984). On the other hand, CaO, Fe₂O₃, P₂O₅ and MnO exhibited higher concentrations compared to these standards, while SiO₂ was much lower. Exceptionally, CaO (27.5 – 71.05%), higher by one order of magnitude than the geochemical standards, indicated a predominance of carbonates in the sedimentary environment of the Ama Fatma site, which should be associated to the abundant bivalve and gastropods fossilized remains. In addition, carbonate precipitation occurs normally in open marine environments characterized by weak clastic inputs in the water. Therefore, the high carbonate content in the black shale of Ama Fatma reveals that deposition occurred in an open marine setting, as reported in previous research for similar environment (Armenteros and Huerta 2006).

Table 2. Concentrations of major elements in the black shale from the Ama Fatma site along with the corresponding averaged values in UCC (Upper Continental Crust), NASC (North American Shale Composite) and PAAS (Post-Archean Australian Shale). Major elements are expressed in %

Table 3. Concentrations of trace elements in the black shale form the Ama Fatma site along with the corresponding averaged values in UCC (Upper Continental Crust), NASC (North American Shale Composite) and PAAS (Post-Archean Australian Shale). Trace elements are in ppm. n.d.means not determined

Pearson correlations among major elements are given in Table 4. Fe₂O₃, Al₂O₃, MgO, Na₂O, TiO₂, P₂O₅ and MnO were found to be negatively correlated to TOC and CaO. The degrees of correlation vary from one element to another and are, in general, moderate to weak. This suggests that these oxides are not associated to organic matter and carbonate contents, but rather to the terrigenous fractions and associated minerals. On the other hand, SiO₂ is positively well correlated to Fe₂O₃ (r = 0, 77), MgO (r = 0, 85), Na₂O (r = 0, 57), TiO₂ (r = 0, 69), P₂O₅ (r = 0, 86) and MnO (r = 0, 56), indicating that these elements are of terrestrial origin and have been probably transported as detrital components. Silicium found in black shale is, in general, of terrestrial origin (Sari et al 2012). The relatively low contents of Al₂O₃ and SiO₂ should be attributed to the low quantity of clay material in our samples. The positive correlation among Al₂O₃, K₂O and SiO₂ indicates that these elements are associated to the clay fraction, and the samples are essentially composed of illite.

Table 4. Pearson’s correlations among major and trace elements

The non-significant negative correlation between P₂O₅ and TOC reveals that the phosphate is not associated to the organic phase. It is well known that this element is often confined in the apatite (Kasper-Zubillaga et al. 2008; Fu et al. 2010b), and cannot precipitate directly with the inorganic component in seawater and, therefore, phosphorous has been incorporated to the sedimentary shale along with the terrigenous substances (Mu 1999). In addition, the strong correlations among Mn, Fe and P are indicative of the presence of H₂S which promote the precipitation of these elements as oxides (Kholodov 2001). This could be behind their relative high concentrations compared to UCC, PAAS and NASC.

3.3. Uraniumand Thorium

U and Th concentrations found in the black shale from Ama Fatma were ranging from 4.09 to 17.84 ppm and from 1.29 to 4.48 ppm with depth averaged values of 13.2 ppm and 2.48 ppm, respectively. U levels are much higher than those reported for the geological standards (UCC, PAAS and NASC) while Th levels are lower. This ultimate was found to be significantly and positively correlated to SiO₂, Fe₂O₃, MgO, K₂O, TiO₂, P₂O₅, and MnO, and not correlated to TOC and CaO indicating that thorium is pre-dominantly of detrital origin. The weak correlation between U and TOC, and the non-correlation with SiO₂ and Fe₂O₃ might be due to the relatively high content of OM which could influence the oxidation state of U, and consequently its solubility/mobility. In addition, organic matter may possibly compete with U in the sorption processes onto mineral surfaces, such as Fe- and Si- oxides and hydroxides (Cumberland et al. 2016).

3.4. Trace Elements Geochemistry and Enrichment

From trace elements abundances given in Table 3, Li, Cr, Cu, In, Sb were found to be of the same range of concentrations of the averages reported for geological standards (UCC, PAAS and NASC), while elements such as Ga, Rb, Zr, Nb, Ba and Th are lower. In contrast, Be, V, Co, Ni, Zn, Sr, Mo, Cd, Pb, Bi and U presented concentrations higher, to much higher, than the standards. On the other hand, all trace elements, except for Mo and Sb, did not appear to be associated to organic matter as can be seen in the correlation table. The negative correlations of some heavy metal elements with CaO could be attributed to the adsorption mainly on clay and/or MnO minerals. Secondly, the heavy metals may enter the calcium lattice defects and, therefore, constitute metal carbonate (Aziz HA et al. 2008).

Element enrichments in our black shale samples were estimated by using the Enrichment Factor (EF) defined as follows (Brumsack 2006):

(1)

Where X and Al are the element and aluminium concentrations. Al has been used as normalizing element to compensate for grain size and mineralogical variability, since it represents alumino-silicates (Luoma and Rainbow 2008). In Table 5, EFs calculated from equation (1) are presented. The studied trace elements were found to be classified in five categories according to the enrichment levels established through the averaged EF throughout the vertical section (Sutherland 2000):

- EF<2, weakly enriched: Ba

- 2<EF<5, moderately enriched: Rb, Zr

- 5<EF<20, significantly enriched: Cu, Ga, Nb, Th, Cr, Sr

- 20<EF<40, highly enriched: Pb, Zn, Ni, Co, V

- EF>40, extremely enriched: U, Mo

The depth-averaged EFs were in the order Ba<Rb<Zr<Cu<Ga<Nb<Th<Cr<Sr<Pb<Zn<Ni<Co<V<U<Mo. Mo showed the highest enrichment because of its links with pyrite and sulfur-rich organic matter (Tribovillard et al. 2004). Therefore, the extreme enrichment in Mo (EF: 793.47) and U (EF: 94.16) and high enrichment in Co, Ni, V, Zn and Pb (EF: 23, 57-32, 70) in our samples suggested that the Ama Fatma bituminous shale was deposited under euxinic conditions.

V and Ni seem to be highly enriched in our samples. Such enrichment occurs usually in bitumen that is associated with type I and II kerogen as suggested by Lewan and Maynard (1982). Therefore, the organic matter contained in the Ama Fatma shale is associated with type II kerogen, in agreement with previous studies carried out on samples from the same site (El Albani 1995; El Batal 2014).

Table 5. Mean enrichment factor (EF) of selected trace elements in the Ama Fatma black shale section. EF = (X/Al) sample/(X/Al) PAAS

3.5. Redox Conditions Reconstruction

Trace elements ratios are widely used as proxies for establishing the redox conditions of the depositional environment in old and modern sedimentary systems (Calvert and Pedersen 1993; Jones and Manning 1994; Wignall 1994; Crusius et al. 1996; Dean et al. 1997, 1999; Yarincik et al. 2000; Morford et al. 2001; Pailler et al. 2002). Indeed, previous studies have used ratios such as V/(V + Ni), V/Cr and U/Th to reconstruct redox conditions under which the geological formations occurred (Hatch and Leventhal 1992; Jones and Manning 1994; Rimmer et al. 2004). In Figure 3, the vertical profiles of the above mentioned ratios are plotted.

Figure 3. Vertical profiles of V/(V+Ni), V/Cr and U/Th in black shale form the Ama Fatma site

According to Lewanand Hatch and Leventhal (1992), V/(V+Ni) ratios greater than 0.84 are indicative of euxinic conditions, while ratios ranging from 0.54 to 0.82 and from 0.46 to 0.60 are typical of anoxic and dysoxic conditions, respectively. V/(V+Ni) ratios between 0.63 and 0.78 were obtained for our black shale samples, pointing toward anoxic conditions during the deposition.

V/Cr ratio has also been used as indicator of redox conditions changes in deposition environments. Vanadium, which is initially incorporated in tetrapyrrole structures, is preferably accumulated in reducing conditions (Calvert and Piper 1984; Tribovillard et al. 2006; Soua 2010). This element can also be adsorbed on clay minerals during burial (Breitand Wanty 1991), while chromium is associated to the detrital and is not biased by the prevailing oxydoreduction conditions (Dill 1986). Ratios of V/Cr greater than 2 are indicative of anoxic condition, while values lesser than 2 suggest oxidative conditions. The ratios found for our samples were all ranging from 2.57 and 7.04 revealing anoxic depositional conditions. The lowest value was recorded at the upper layer S1(9) followed by an increase to a maximum at S1(14) and then a decrease. Such substantial variability throughout the shale rock column should be attributable to changes in redox potential during deposition.

U/Th ratio has also been used as excellent indicator of redox conditions due to different behavior of U and Th in terms of solubility in aqueous suspensions (Dill 1986; Jones and Manning 1994; Rimmer et al. 2004). Indeed, U(IV), in oxygen depleted environments, is reduced to its insoluble form U(IV) which is predominantly adsorbed onto particles (Algeo and Maynard 2004; Tribovillard et al. 2006). In contrast, Th is known to be a high reactive-particle element and, therefore, remains practically associated to clay minerals, insoluble debris or heavy minerals (Anderson et al. 1983). Consequently, high U/Th ratios (>1.25) are indicative of anoxic conditions, while values lesser than 0.75 are characteristic of oxic environments (Jones and Manning 1994). The U/Th values found in our samples were all ranging from 2.47 to 11.51, revealing that the corresponding black shale were deposited while anoxic conditions were prevailing, being in good agreement with the previous results using V/(V+Ni) and V/Cr as proxies of redox conditions.

4. Conclusions

In the present work, the geochemical characterization of the black shale from the Ama Fatma coastal site was investigated as a preliminary step for the use of this geological matrix in the storage of radioactive wastes. The results showed that:

1. Particle size distribution in black shale are dominated by sand followed by silt and then clays, giving our black shale a sandy loam texture;

2. The mineralogical composition of Ama Fatma black shale is dominated by calcite with a low fraction of dolomite, Quartz, clays, halite, fluorapatite and pyrite;

3. Major element distributions showed higher concentration of CaO followed by SiO₂ and Fe₂O₃, while the other elements were less abundant;

4. The high carbonate content in the Ama Fatma black shale reveals that deposition occurred in an open marine setting;

5. The correlation among major elements of our samples indicates that these oxides are associated to the terrigenous fractions and associated minerals;

6. U and Th concentrations were found to be controlled by organic matter content and detrital component;

7. Trace elements distribution show a high enrichment in Mo, U, Co, Ni, V, Zn and Pb indicating that the Ama Fatma bituminous shale was deposited under euxinic conditions;

8. The high enrichment in V and Ni suggested that our bituminous shale are associated with type I and II kerogen;

9. Trace elements ratios revealed an anoxic condition deposition of our studied oil shale.

ACKNOWLEDGEMENTS

This work has been carried out within the framework of the UMR between the University Hassan 2 - Casablanca and the National Center for Energy, Nuclear Science and Technology, and in the framework of the collaboration held between the Faculty of Sciences-Rabat and the University of Seville - Spain.